1. Field of the Invention
The present invention relates to a method for immersing a wafer in a solution.
2. Description of the Prior Art
Wet etching is the most commonly used method in the manufacturing of a semiconductor wafer. This method requires immersion of the semiconductor wafer in an organic solution followed by etching. How etching is performed depends on the chemical reaction between the film on the surface of the wafer and the solution. The outcome of the reaction affects the yield and is highly dependent on how evenly exposed to solution the wafer is.
Usually, several semiconductor wafers are simultaneously immersed into an organic solution. The reactants in the solution pass by diffusion through the interface boundary layer to reach the surface of the film to be etched where they react chemically with constituent molecules on the surface of the film to produce various gaseous and liquid chemical compounds. Etching of the surface of the film is caused by the mass depletion of the constituent molecules. The products of the reaction diffuse back into solution and remain in the organic solution after etching is completed.
The primary factors affecting wet etching are the concentration of the solution, the temperature of the reaction, the period of reaction, and the type of stirring. If the concentration of the organic solution or the temperature of the reaction is high, then the rate of the etching will be high. But if the reaction is too fast, it will cause serious undercut and that part of the film to be preserved may become inadvertently etched. On the other hand, if the rate of the reaction is slow, the overall etching process will take more time. Proper stirring produces convection in the organic solution which reduces the thickness of the interface boundary layer. This facilitates the movement of reactants to the surface of the film and increases the rate of etching. Certain stirring methods such as ultrasonic agitation or bubbling can minimize the undercut phenomena. However, improper stirring of the organic solution will affect the stability of the whole reaction.
Please refer to FIG. 1. FIG. 1 is a schematic diagram of a semiconductor wafer 10. A semiconductor wafer 10 usually contains as many as several hundred integrated chips 12 arranged uniformly on its surface. When wet etching, it is necessary to carefully modify the etching parameters mentioned above to make all the chips 12 react properly and to provide a uniform etching environment for each chip 12. If each chip 12 is immersed in the organic solution for a different length of time or comes in contact with organic solutions of different concentrations, then each chip 12 will be etched to a different extent and the yield of the process will decrease.
In the manufacturing of semiconductor wafers, dozens of wafers 10 are immersed simultaneously into a solution chamber, left in the chamber for a predetermined period of time, then removed. This process is automatic. As the wafer 10 enters the chamber, its bottom part comes into contact with the solution first. The bottom part of the wafer 10 is also the last to leave the solution during removal from the chamber. Therefore, different segments of the wafer are exposed to the solution for different periods of time causing variation in the extent of etching. It can be seen from this example that the bottom part of the wafer 10 is more extensively etched than the top part which is immersed for a shorter period of time in the solution. Therefore, chips 12 on the various regions of the wafer are also etched to varying extents. This seriously affects the yield of the processing of the wafer 10.
In order to solve the yield problem caused by different etching of the chips 12 on the same wafer, two methods are currently used by the semiconductor industry. One is to speed up the process of putting the wafer into the solution chamber and pulling it out, the other is to decrease the concentration of the organic solution. Increasing the speed of movement will decrease the differences of immersion time, but the fast motion will cause non-uniform stirring of the solution. Decreasing the concentration of the organic solution will lower the reaction rate and, hence variation in the period of the reaction for different parts of the wafer 10 will not substantially influence the extent of etching. But low reaction rate will increase the length of time required for producing the semiconductor wafer.
It is therefore a primary objective of the present invention to provide a method for evenly immersing a wafer in a solution to solve the problem in wet etching mentioned above.
In a preferred embodiment, the present invention provides a method for evenly immersing a wafer in a solution comprising:
(1) placing at least one disk-shaped wafer inside a wafer holder which is used for vertically holding at least one wafer;
(2) immersing the wafer holder into the solution vertically so that each wafer in the wafer holder can be vertically immersed into and react with the solution;
(3) vertically rotating the wafer holder in the solution so as to invert each wafer in the wafer holder upside down; and
(4) removing the wafer holder from the solution vertically after immersing the wafer in the solution for a predetermined period of time.
It is an advantage of the present invention that each portion of the semiconductor wafer is evenly immersed into solution for the same length of time so as to improve the yield of the process. Since there is no difference in the immersion time for each part of the wafer, the density of the organic solution may be increased to increase the reaction rate. This leads to an increase in the overall production rate.
This and other objectives of the present invention will no doubt become obvious to those of ordinary skill in the art after having read the following detailed description of the preferred embodiment which is illustrated in the various figures and drawings.